The present invention relates to automotive vehicle speed control systems and to such systems that track a target vehicle through a curve.
Adaptive cruise control (ACC) systems are gaining wide spread acceptance in the automotive industry. Adaptive cruise control systems utilize a conventional cruise control system, which maintains a desired vehicle speed. In addition, the adaptive cruise control system can modify the speed of the vehicle to accommodate for changes in the traffic conditions. The ACC system accomplishes this through automatic acceleration, deceleration and/or braking. Thus, the vehicle having the ACC (host vehicle) maintains a safe distance from the vehicle driving in front (target vehicle) of the host vehicle as a function of road speed.
Typically prior art adaptive cruise control systems include an adaptive cruise control processor, a radar sensor, a brake intervention system, a display unit, an engine intervention system, a plurality of sensors (i.e., wheel speed, yaw rate, steering wheel angle, lateral acceleration), and a transmission intervention system. Generally, the radar sensor operates at a frequency of 76 to 77 (GHz), which was specifically allocated for ACC system. In operation, a radar beam is emitted from the host vehicle and is reflected from the target vehicle back toward the host vehicle. An analysis of the emitted and reflected radar waves is conducted to determine propagation time, Doppler shift, and amplitude of the waves for pulse radar, and beat frequency and Doppler shift for FMCW radar. From this analysis, the distance, relative speed and relative azimuth angle with respect to the host vehicle is calculated.
One significant problem for the ACC to overcome is to ensure reliable operation of the system in varying situations such as entering curves in the road or lane changes. For proper system operation, it is essential that the target vehicle is correctly identified and the lane change and curve-enter/-exit are distinguished from each other. Prior art systems obtain information typically from a yaw rate sensor, a steering wheel angle sensor, a wheel speed sensor, and typically a lateral speed sensor to determine the target vehicle""s lane location and curve status. Other systems under consideration for determining vehicle location on a roadway are video imaging systems.
The problem of determining the target vehicle""s lane location becomes more complex when the target vehicle has entered a curve and the host vehicle has not yet entered the curve or vice versa. Prior art methods using steering angle and yaw rate information do not properly address this transient condition. Thus, when the host vehicle is following a target vehicle that has entered a curve, the ACC sometimes loses the target, especially when the host vehicle is on a straight course and the target vehicle enters the curve. Essentially, prior art systems do not effectively judge whether the target vehicle is in the same lane as the host vehicle or whether the host vehicle has entered the curve or vice versa. Specifically, the situation in which the target vehicle enters the curve may be confused with the situation in which the target vehicle changes lanes.
Therefore, what is needed is a new and improved method for determining whether a target vehicle has entered a curve or whether the target vehicle has changed lanes. This new method should accurately predict the location of the target vehicle with only host vehicle and radar information, and does not require extensive experimental data for adaptation of parameters. Moreover, the new method for tracking the target vehicle should prevent the loss of the target vehicle as the target vehicle enters and exits a curve in a roadway.
In an aspect of the present invention a method for determining whether a target vehicle is in the path of the host vehicle is provided. The method includes using an azimuth angle and calculating relative velocity between a target vehicle and a host vehicle at a predefined time interval. Further, a determination of whether a target vehicle is in the same lane as the host vehicle (in the path of the host vehicle) is evaluated. The candidate target vehicle having the shortest range is then identified. However, if there is no candidate vehicle then other preceding target vehicles are identified. Again, if the preceding vehicle is identified, the vehicle having the shortest range relative to the host is determined to be the primary target vehicle. A curve is fit to the measured data using the following theoretical relationship:
xcex8=(R/2L)(vr/V)2 
xcex8=xe2x88x92(R/2L)(vr/Vxe2x88x92L/R)2+L/2R 
where:
V=velocity of host vehicle;
r=the absolute value of the relative velocity;
xcex8=the azimuth angle; and
vri=the absolute value of the measured relative velocity.
After a curve (Y=a1 X+a2 X2) is fit to the measured data (xi, yi)=(vri/V, xcex8), the following equations are used to determine whether the target vehicle is entering or exiting a curve. If for example, equation: |"sgr"a2/a2| less than xcex4 (16) where "sgr"a2 is standard deviation of the regression coefficient a2 and xcex4=0.25 or other appropriate predefined value, is satisfied then one of the target vehicle and host vehicle is determined to be on a curve. It is then determined whether the following equation: |a1/a2| less than  less than L/|R| where: R is the radius of curvature estimated from the regression and L is the range between the host and target vehicle, is satisfied. If equation is satisfied the target vehicle is determined to be at the entrance of a curve. However, if the equation is not satisfied, then the target vehicle is determined to be at the exit of the curve.
These and other aspects and advantages of the present invention will become apparent upon reading the following detailed description of the invention in combination with the accompanying drawings.